Power converters

ABSTRACT

A power converter that interfaces a motor requiring variable voltage/frequency to a supply network providing a nominally fixed voltage/frequency includes a first rectifier/inverter connected to a stator and a second rectifier/inverter. Both rectifier/inverters are interconnected by a dc link and include switching devices. A filter is connected between the second rectifier/inverter and the network. A first controller for the first rectifier/inverter uses a dc link voltage demand signal indicative of a desired dc link voltage to control the switching devices of the first rectifier/inverter. A second controller for the second rectifier/inverter uses a power demand signal indicative of the level of power to be transferred to the dc link from the network through the second rectifier/inverter, and a voltage demand signal indicative of the voltage to be achieved at network terminals of the filter to control the switching devices of the second rectifier/inverter.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of U.S. patent applicationSer. No. 12/151,118, filed May 2, 2008, now allowed.

FIELD OF THE INVENTION

The present invention relates to power converters, and in particular topower converters that can be used to interface motors operating atvariable voltage at variable frequency to a power grid or supply networkat nominally fixed voltage and frequency. The present invention alsoincludes features that allow the power converters to remain connected tothe supply network and retain control during network fault and transientconditions. The power converters are particularly suitable for use with,but not restricted to, marine propulsion systems.

BACKGROUND OF THE INVENTION

The propeller assembly of a marine vessel typically rotates at variablespeed, both in the forward and reverse directions. Where a marine vesseluses an electric power and propulsion system then the motor driving thepropeller assembly will also need to operate at variable frequency (forthe case where the propeller assembly is driven directly by the rotor ofthe motor then the frequency will be approximately proportional to thespeed of rotation of the propeller assembly) and variable voltage(approximately proportional to frequency). The power system of themarine vessel will have a nominally fixed voltage and frequency and itis therefore necessary to interface the motor to the power system usinga power converter.

The power converter will typically consist of two parts: a networkbridge that rectifies the ac power from the power system to a nominallyfixed dc voltage (the dc link), and a motor bridge that inverts the dcvoltage to the appropriate ac voltage for the motor. The power systemsof many marine vessels are often referred to as being “weak” since thetotal generating capacity is closely matched to the total load. Thismeans that when large loads connected to the power and propulsion systemare turned on, significant transients (dips) can occur. Any dips orfaults on the power system will interfere with the network bridge andits operation to provide the dc voltage. It is therefore normal for thepower converter to be turned off to avoid damaging the variouscomponents. For many marine applications this requirement to turn offthe power converter, and hence the total loss of the associatedpropulsion equipment, is considered unacceptable.

There is therefore a need for an improved power converter that canremain connected to the power system in the event of a dip or a fault.

SUMMARY OF THE INVENTION

The present invention provides a power converter that can be used tointerface a motor that requires variable voltage at variable frequencyto a supply network providing a nominally fixed voltage and nominallyfixed frequency, the power converter comprising:

-   -   a first active rectifier/inverter electrically connected to the        stator of the motor and including a plurality of semiconductor        power switching devices;    -   a second active rectifier/inverter electrically connected to the        supply network and including a plurality of semiconductor power        switching devices;    -   a dc link connected between the first active rectifier/inverter        and the second active rectifier/inverter;    -   a first controller for the first active rectifier/inverter; and    -   a second controller for the second active rectifier/inverter;    -   wherein the first controller uses a dc link voltage demand        signal indicative of a desired dc link voltage to control the        semiconductor power switching devices of the first active        rectifier/inverter to achieve the desired level of dc link        voltage that corresponds to the dc link voltage demand signal;        and    -   wherein the second controller uses a first demand signal to        control the semiconductor power switches of the second active        rectifier/inverter to control the level of real power to be        transferred into and out of the dc link through the second        active rectifier/inverter, and a second demand signal to control        the semiconductor power switching devices of the second active        rectifier/inverter to control the level of reactive power and/or        ac voltage at the supply network or, if a filter is connected        between the second active rectifier/inverter and the supply        network, at the network terminals of the filter.

The power converter can be used to interface the motor to the supplynetwork during situations where the supply network is operatingnormally, but also includes features that allow it to operate insituations where the supply network voltage is varying due to faults ortransients on the supply network. The second controller is also able tocontrol the second active rectifier/inverter to provide voltage supportto the supply network when the supply network voltage deviates from itsnominal condition.

The motor can be a rotating or linear motor of any suitable type.Examples would include an induction motor or a synchronous motor(excited by any suitable means such as permanent magnets or conventionalor superconducting windings). In the case of a rotating motor, the rotorcan be connected to a propeller assembly or any other mechanical loadrequiring ride-through during transients or faults. However, themechanical load must normally retain sufficient energy (in the form ofinertia, momentum etc.) to maintain operation during the time when it isnot possible to obtain electrical power from the supply network. Alinear motor could be used in applications that require reciprocatingmotion.

In the case where the motor is an induction motor, the first controllerpreferably uses a flux demand signal indicative of a desired level offlux to be achieved in the motor to control the semiconductor powerswitching devices of the first active rectifier/inverter to producestator electrical quantities that achieve the desired flux in the motor.The term “stator electrical quantities” is used herein to refer to anyand all of the individual phase voltage magnitude individual phasecurrent magnitude, phase and frequency in a multi-phase motor.

The operation of the semiconductor power switching devices in the firstactive rectifier/inverter can be controlled using gate drive controlsignals derived in accordance with a conventional pulse width modulation(PWM) strategy.

The first controller preferably compares the dc link voltage demandsignal indicative of a desired dc link voltage to a dc link voltagefeedback signal to determine a torque demand signal indicative of adesired level of torque to be achieved in the motor. The firstcontroller can then control the semiconductor power switching devices ofthe first active rectifier/inverter to produce stator electricalquantities that achieve the desired torque in the motor. The controlimplementation described in more detail below is based on vectorcontrol, which is a well-known and commonly used technique. However, itwill be readily appreciated that any other suitable method of controlimplementation (such as discrete time field oriented control (DT-FOC) ordirect torque control, for example) could be used instead.

The first controller preferably supplies a control signal that varies inaccordance with the prevailing motor conditions to the secondcontroller. This control signal can then be used by the secondcontroller to limit the level of power that is transferred to the dclink from the supply network through the second activerectifier/inverter. The control signal therefore prevents any more powerbeing imported into the dc link through the second activerectifier/inverter when the motor has reached its physical performancelimits (i.e., when the motor is already operating at its maximum shaftspeed or its maximum rate of acceleration, for example).

The first demand signal that is used to control the level of real powerto be transferred into and out of the dc link through the second activerectifier/inverter is preferably based on a motor demand (e.g., anoperating demand for the motor) and the second controller can use thefirst demand signal to produce desired motor electrical and mechanicalquantities. The term “motor electrical and mechanical quantities” isused herein to refer to any and all of the motor flux, current, voltage,torque, speed and power of the motor.

The second controller preferably uses the first demand signal todetermine a quadrature axis current demand signal for the second activerectifier/inverter indicative of a desired quadrature axis current to beachieved in the supply network.

The second controller can then control the semiconductor power switchingdevices of the second active rectifier/inverter to produce filter/supplynetwork electrical quantities that achieve the desired quadrature axiscurrent in the supply network.

The first demand signal can be provided by a suitable controller. Thecontroller can receive one or more of a power reference, a speedreference, torque reference, and a current reference from a vesselcontrol system (which can optionally incorporate a dynamic positioningsystem). The reference(s) can also be provided directly from controllevers of the marine vessel or the like.

The second controller can use the second demand signal to control thesemiconductor power switching devices of the second activerectifier/inverter to produce filter/supply network electricalquantities. The term “filter/supply network electrical quantities” isused herein to refer to any and all of the individual phase voltagemagnitude, individual phase current magnitude, phase and frequency in amulti-phase active rectifier/inverter system. The term “multi-phase”typically refers to three-phase but can include other numbers of phases.The operation of the semiconductor power switching devices in the secondactive rectifier/inverter can also be controlled using gate drivecontrol signals derived in accordance with a conventional PWM strategy.

The second controller preferably uses the second demand signal todetermine a direct axis current demand signal for the second activerectifier/inverter. The second controller can then control thesemiconductor power switching devices of the second activerectifier/inverter to produce filter/supply network electricalquantities that achieve the desired direct axis current in the supplynetwork.

The second controller can modify the direct axis current demand signalin accordance with the prevailing supply network voltage conditions.

A signal indicative of the supply network power is preferably suppliedto the first controller from the second controller. The signalindicative of the supply network power can be added to the output of adc link controller in the first controller and used to determine adesired level of torque in the motor. The signal effectively provides anadvance warning to the first controller that more or less power is goingto be imported into the dc link through the second activerectifier/inverter. The first controller can then start to determine thedesired level of torque in the motor before the change in the amount ofimported power causes a corresponding change in the dc link voltage.This can be important for transient reasons.

The present invention also provides an arrangement comprising aplurality of power converters as described above connected to a commonsupply bus of a supply network providing a nominally fixed voltage andnominally fixed frequency, wherein the second demand signal is suppliedto the second controller of each power converter from a power managementsystem.

Each individual power converter preferably includes a step-downtransformer electrically connected between the associated filter and thecommon supply bus.

The power converter is particularly suitable for use in a propulsionunit for use in marine vessels. The present invention therefore alsoprovides a propulsion unit comprising a motor having a stator and rotor,a propeller assembly including at least one blade rotated by the rotorof the motor, and a power converter as described above. The propellerassembly can be integral with the rotor of the motor. Alternatively, thepropeller assembly is mounted to a rotatable shaft and the rotor of themotor is coupled to the rotatable shaft either directly or indirectlythrough a gearbox.

A plurality of propulsion units can be used in a marine vessel. Thepresent invention therefore also provides a marine vessel comprising asupply network providing a nominally fixed voltage and nominally fixedfrequency and having a common supply bus, and a plurality of propulsionunits described above. The respective power converters of the pluralityof propulsion units are connected to the common supply bus and whereinthe second demand signal is supplied by a power management system.

The present invention further provides a method of operating a powerconverter that can be used to interface a motor that requires variablevoltage at variable frequency to a supply network providing a nominallyfixed voltage and nominally fixed frequency, the power convertercomprising:

-   -   a first active rectifier/inverter electrically connected to the        stator of the motor and including a plurality of semiconductor        power switching devices;    -   a second active rectifier/inverter electrically connected to the        supply network and including a plurality of semiconductor power        switching devices;    -   a dc link connected between the first active rectifier/inverter        and the second active rectifier/inverter;    -   a first controller for the first active rectifier/inverter; and    -   a second controller for the second active rectifier/inverter;    -   wherein the method comprises the steps of:    -   the first controller using a dc link voltage demand signal        indicative of a desired dc link voltage to control the        semiconductor power switching devices of the first active        rectifier/inverter to achieve the desired level of dc link        voltage that corresponds to the dc link voltage demand signal;        and    -   the second controller using a first demand signal to control the        semiconductor power switches of the second active        rectifier/inverter to control the level of real power to be        transferred into and out of the dc link through the second        active rectifier/inverter, and a second demand signal to control        the semiconductor power switching devices of the second active        rectifier/inverter to control the level of reactive power and/or        ac voltage at the supply network or, if a filter is connected        between the second active rectifier/inverter and the supply        network, at the network terminals of the filter.

The method can include further steps as outlined below.

The first controller can use a flux demand signal indicative of adesired level of flux to be achieved in the motor to control thesemiconductor power switching devices of the first activerectifier/inverter to produce stator electrical quantities that achievethe desired flux in the motor.

The first controller can compare the dc link voltage demand signalindicative of a desired dc link voltage to a dc link voltage feedbacksignal to determine a torque demand signal indicative of a desired levelof torque to be achieved in the motor, and control the semiconductorpower switching devices of the first active rectifier/inverter toproduce stator electrical quantities that achieve the desired torque inthe motor.

The first controller can supply a control signal that varies inaccordance with the prevailing motor conditions to the secondcontroller. The second controller can then use the control signal tolimit the level of power that is transferred to the dc link from thesupply network through the second active rectifier/inverter.

The first demand signal can be based on a motor demand and the methodcan further comprise the step of the second controller using the firstdemand signal to produce desired motor electrical and mechanicalquantities.

The second controller can use the first demand signal to determine aquadrature axis current demand signal for the second activerectifier/inverter indicative of a desired quadrature axis current to beachieved in the supply network, and control the semiconductor powerswitching devices of the second active rectifier/inverter to producefilter/supply network electrical quantities that achieve the desiredquadrature axis current in the supply network.

The first demand signal can be provided by a suitable controller. Thecontroller can receive one or more of a power reference, a speedreference, a torque reference, and a current reference from a vesselcontrol system.

The second controller can use the second demand signal to control thesemiconductor power switching devices of the second activerectifier/inverter to produce desired filter/supply network electricalquantities.

The second controller can use the second demand signal to determine adirect axis current demand signal for the second activerectifier/inverter, and control the semiconductor power switchingdevices of the second active rectifier/inverter to produce filter/supplynetwork electrical quantities that achieve the desired direct axiscurrent in the supply network during a supply network voltage dipsituation. The use of the word “dip” in this description in relation tosupply network dip situations refers to a situation where the supplynetwork voltage is reduced below its nominal value as a result of eithersymmetrical or asymmetrical network fault conditions, or simply throughthe switching of a large inductive component (such as a transformer orharmonic filter, for example) connected to the supply network.

The second controller can modify the direct axis current demand signalin accordance with the prevailing supply network voltage conditionsduring a supply network voltage dip situation.

The second demand signal can be provided by a power management system.

The second controller can supply a signal indicative of the supplynetwork power to the first controller. The signal indicative of thesupply network power can be added to the output of a dc link controllerin the first controller and used to determine a desired level of torquein the motor.

The present invention also provides a method of operating a plurality ofpower converters as described above connected to a common supply bus ofa supply network providing a nominally fixed voltage and nominally fixedfrequency, the method comprising the step of supplying the second demandsignal indicative of the voltage to be achieved at the supply network,or the network terminals of the filter of each power converter, to thesecond controller of each power converter from a power managementsystem.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments of the invention will now be described, withreference to the accompanying drawings, in which:

FIG. 1 is a schematic drawing showing how a power converter according tothe present invention is used to interface between a motor and a supplybus of a fixed frequency supply network; and

FIG. 2 is schematic drawing showing how a number of power convertersaccording to the present invention can be connected to the supply bus aspart of a marine propulsion system.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Power ConverterTopology

The basic topology of the power converter will be outlined withreference to FIGS. 1 and 2. Although the power converter is describedbelow in the context of electric marine propulsion systems, it will bereadily appreciated that other uses are possible. For example, the powerconverter can be used as part of a drive system for pumps, fans,compressors or other industrial type loads.

A propeller assembly 2 of an electric marine propulsion system is drivenby the rotor (not shown) of a variable speed ac induction motor 4. Thepropeller assembly 2 will normally consist of a number of blades mountedon a rotating shaft with a fixed pitch. The rotating shaft can bedirectly connected to the rotor of the motor 4 or indirectly through agearbox (not shown) that is used to adjust the shaft speed. The speed atwhich the propeller assembly 2 must rotate will depend on the speed ofthe marine vessel and the level or direction of thrust required forpropulsion. However, because the speed of rotation varies, the voltageand frequency applied to the terminals of the motor 4 must also vary.

The terminals of the motor 4 are connected to the ac terminals of athree-phase motor bridge 10 which in normal operation operates as aninverter to supply power to the motor from a dc link 12. The motorbridge 10 has a conventional three-phase two-level topology with aseries of semiconductor power switching devices fully controlled andregulated using a pulse width modulation (PWM) strategy. However, inpractice the motor bridge 10 can have any suitable topology such as athree-level neutral point clamped topology or a multi-level topology.The derivation of the gate drive command signals that are used tocontrol the semiconductor power switching devices of the motor bridge 10is described in more detail below.

A supply network of the marine vessel (FIG. 2) operates at a nominallyfixed frequency and includes a common ac supply bus (labelled “bus”)that receives power from an ac generator 28. Power is supplied to the acterminals of a network bridge 14 from the supply bus via a step-downtransformer 6 and is filtered by inductors 16 (and other possiblefilters). Protective switchgear (not shown) can be included to provide areliable connection to the supply bus and to isolate the propulsionsystem from the supply network for various operational andnon-operational requirements.

The network bridge 14 in normal operation operates as an activerectifier to supply power from the supply bus to the dc link 12. Thenetwork bridge 14 has a similar three-phase two-level topology to themotor bridge 10 with a series of semiconductor power switching devicesfully controlled and regulated using a PWM strategy. However, inpractice the network bridge 14 can have any suitable topology asdiscussed above for the motor bridge 10. The principal control for thedc input voltage of the motor bridge 10 is achieved by controlling themotor bridge. The network bridge 14 is controlled to meet two principalobjectives, namely active power and network voltage. A detaileddescription of how this control is achieved is provided below.

The derivation of the gate drive command signals that are used tocontrol the semiconductor power switching devices of the network bridge14 is also described in more detail below.

As described herein, active rectification (as the prime mode ofoperation of the network bridge 14) is the conversion of energy from theac terminals of the three-phase network bridge to the dc link andinversion (as the prime mode of operation of the motor bridge 10) is theconversion of energy from the dc link of the three-phase motor bridge toits ac terminals. However, it will be readily appreciated that there canbe times when it might be necessary or desirable to operate the motorbridge 10 as an active rectifier and the network bridge 14 as aninverter. For example, in a situation where the marine vessel is brakingthen the propulsion system can be adapted to support regeneration. Inthis case the motor 4 can be operated in a generating mode to providepower back to the supply network (or a dump resistor) through the motorbridge 10 operating as an active rectifier and the network bridge 14operating as an inverter.

In situations where a supply network voltage dip occurs, the networkbridge 14 can operate in either an active rectifier mode or in aninverter mode as required in order to provide reactive current into thesupply network to counteract the cause of the voltage dip.

Marine Propulsion System Topology

As mentioned briefly above, a number of power converters can beconnected to the common ac supply bus of the nominally fixed frequencysupply network to define a marine propulsion system. This is shownschematically in FIG. 2. Each power converter 1 a to 1 d includes anassociated filter 16 a to 16 d and a step-down transformer 6 a to 6 d.

In a conventional marine propulsion system, the desired supply networkvoltage would typically be set by a power management system 26 andprovided to the automatic voltage regulator (AVR) 30 of each generator28. In the arrangement of FIG. 2, the power management system 26 alsosupplies a voltage demand signal VBUS* to each of the power converters 1a to 1 d. The voltage demand signal VBUS* represents the desired voltageto be achieved at the network terminals of each filter 16 a to 16 dduring normal operation of the power converter.

A large inductive component in the form of a transformer 32 is alsoconnected to the common supply bus as shown in FIG. 2.

Motor Bridge Control

The motor bridge controller 18 supplies gate drive command signals tothe motor bridge 10 which cause the semiconductor power switchingdevices to be switched on and off resulting in a particular voltagebeing applied to the terminals of the motor 4.

The motor bridge controller 18 will select the voltage to be appliedbased on a flux demand signal Φ* that represents a desired level of fluxto be achieved in the motor 4, a dc link voltage demand signal VDC_MOT*that represents a desired dc link voltage, a dc link voltage feedbacksignal VDC_FB that is indicative of the actual dc link voltage, one ormore feedback signals derived from a measurement of the motor phasecurrents IU, IV and IW, and a power feedforward signal POWER_FF thatrepresents an indication of the supply network power. The flux demandsignal Φ* and the dc link voltage demand signal VDC_MOT* will usually beset constants for a particular drive configuration. The differencebetween the dc link voltage demand signal VDC_MOT* and the dc linkvoltage feedback signal VDC_FB is used to provide a torque demand signalfor the motor 4. A suitable control implementation such as a vectorcontrol scheme can use the derived torque demand signal and the fluxdemand signal Φ* to generate the appropriate switching pattern for thesemiconductor power switching devices of the motor bridge 10.

It will be readily appreciated that the flux demand signal Φ* can beomitted if the ac motor 4 is a synchronous electrical machine. In thiscase, the switching pattern for the semiconductor power switchingdevices of the motor bridge 10 will be generated solely on the basis ofthe derived torque demand signal.

Network Bridge Control

The network bridge controller 20 supplies gate drive command signals tothe network bridge 14 which cause the semiconductor power switchingdevices to be switched on and off resulting in a particular voltagebeing applied to the filter terminals. The network bridge controller 20will control the level of real power to be transferred into and out ofthe dc link 12 based on a power demand signal P* that is provided by acontroller 24. The power demand signal P* can be derived by thecontroller 24 using any suitable reference such as a speed reference Nand/or a power reference P. The network bridge controller 20 will alsocontrol the voltage to be achieved at the network terminals of thefilter 16 based on the voltage demand signal VBUS* that is provided bythe power management system 26. Additional signals used by the networkbridge controller include one or more feedback signals (V_NET and I_NET)derived from voltage measurements VR, VY and VB (that is the three-phasevoltage measurements taken across the so-called red (R), yellow (Y) andblue (B) output lines that supply power to the network bridge 14 fromthe common supply bus) and current measurements of the network bridgephase currents IR, IY and IB, and a control signal IDC_LIM that is usedto limit the level of power that is transferred to the dc link from thecommon supply bus.

The power demand signal P* is divided by the prevailing quadrature axisnetwork voltage VQ_NET to obtain a quadrature axis current demand signal(i.e. a real current demand signal). The power demand signal P* is alsocompared against the voltage demand signal VBUS* to derive a reactivecurrent demand signal. A suitable control implementation such as avector control scheme can use the derived real and reactive currentdemand signals to generate the appropriate switching pattern for thesemiconductor power switching devices of the network bridge 14.

Operation of the Marine Propulsion System

One possible operational implementation of the above marine propulsiontopology is as follows. At start up the shaft speed is zero and a dclink voltage demand signal VDC_NET* supplied to the network bridgecontroller 20 is set to 950 volts. The dc link voltage demand signalVDC_NET* is only used during start up and will usually be a set constantfor a particular drive configuration. The semiconductor power switchingdevices in the network bridge 14 are enabled and under control of thenetwork bridge controller 20 bring the dc link voltage (VDC) up to 950volts.

At the same time, the dc link voltage demand signal VDC_MOT* applied tothe motor bridge controller 18 is set to 1000 volts.

Assuming that the marine propulsion system has a requirement to providethrust for propulsion of the marine vessel, the semiconductor powerswitching devices in the motor bridge 10 are enabled and the motorbridge controller 18 will control the direct axis current to achieve thenecessary flux in the motor 4. While the dc link voltage is less thanthe dc link voltage demand signal VDC_MOT* a dc link controller (notshown) in the motor bridge controller 18 will request a negativequadrature axis current in the motor 4 (to return power to the dc link12) but this will be blocked because the shaft of the propeller assembly2 is not rotating and there is no shaft power available.

When a thrust requirement is made to the marine propulsion system thiswill either be supplied to the controller 24 as a power reference Pand/or as a speed reference N. The power and/or speed references can beprovided to the controller 24 directly from control levers on the bridgeof the marine vessel or from a vessel control system, for example, andare labelled on FIG. 1 as vessel control commands. A speed reference Ncan be converted to the power demand signal P* by the controller 24 aspart of a speed control loop with reference to the actual speed of themotor 4 detected by a speed sensor.

Applying the power demand signal P* to the network bridge controller 20will cause the dc link voltage to increase. Once the dc link voltagereaches the level set by the dc link voltage demand signal VDC_MOT*, thedc link controller of the motor bridge controller 18 will begin torequest positive quadrature axis current in an attempt to limit the dclink voltage at the desired set level and will start to accelerate theshaft of the propeller assembly 2.

While the dc link voltage is greater than the dc link voltage demandsignal VDC_NET* the voltage control in the network bridge controller 20is disabled.

The magnitude of the power transfer through the network bridge 14 islimited by a signal derived from the power demand signal P*.

Once an initial steady state has been achieved, the power converteroperates in a dynamic manner to accommodate changing thrustrequirements. For example, for an increasing thrust requirement (i.e.,for an increasing power demand signal P*) the network bridge controller20 causes the network bridge 14 to import more power from the supplynetwork to the dc link 12. Increasing the amount of power that isimported to the dc link 12 leads to an increase in the dc link voltage.The motor bridge controller 18 responds to this increase in the dc linkvoltage to cause the motor bridge 10 to draw more power out of the dclink 12 and provides this to the motor 4 until a new steady state isachieved (i.e., where the amount of power that is supplied from thesupply network to the dc link 12 is equal to the amount of power that issupplied from the dc link to the motor 4). In this steady state, the dclink voltage has matched the dc link voltage demand signal VDC_MOT*. Fora reducing thrust requirement then opposite control actions take place.

The supply network shown in FIG. 2 is a “weak network” since the totalgenerating capacity is closely matched to the total load. In the eventof a supply network voltage dip (caused, for example, by the turn-on ofthe transformer 32) the power converter will detect this reducedvoltage, which is seen as a change in the feedback signal V_NET derivedfrom three-phase voltage measurements VR, VY and VB and the networkbridge controller 20 will set an appropriate reactive current demandsignal to supply reactive current back into the common supply bus. Thiswill help to compensate for the current that is being drawn by thetransformer 32 and restore the voltage in the supply network.

In the event of a severe fault somewhere on the common supply buscausing the supply network voltages to be severely reduced then thepower converter will set the power demand signal P* to zero and willsupply reactive current back to the common supply bus until such time asthe supply network voltage has recovered. (In practice, this can beachieved by the network bridge controller 20 effectively overriding thepower demand signal P* supplied by the speed/power controller 24 whilethe fault persists.) During this time, the dc link voltage is sustainedby the motor bridge 10 and the kinetic energy in both the motion of thepropeller assembly 2 and the momentum of the marine vessel.

In order to improve the transient response of the power converter, twocontrol signals are passed between the motor bridge and network bridgecontrollers. More particularly, the network bridge controller 20provides the motor bridge controller 18 with a power feedforward signalPOWER_FF that represents an indication of the amount of power that isbeing transferred into the dc link 12 through the network bridge 14. Thepower feedforward signal POWER_FF therefore provides the motor bridgecontroller 18 with advance notice of a change in the level of power flowbefore the dc link voltage actually starts to increase or decrease. Themotor bridge controller 18 provides a control signal IDC_LIM thatspecifies a dc link current limit. This control signal provides thenetwork bridge controller 20 with a limit as to how much additionalcurrent the motor bridge 18 can accept based on the current shaft speedand acceleration limits of the motor 4.

Practical Implementations of the Marine Propulsion Topology

The marine propulsion topology arrangement can be implemented asfollows. The motor bridge 18 and network bridge 14 can each beimplemented using a MV3000 liquid cooled DELTA inverter module ofsuitable power rating. This is an IGBT-based voltage source invertermodule suitable for operating with ac voltages up to 690 V. The motorbridge controller 18 and the network bridge controller 20 can each beimplemented using a PEC controller. This is a microprocessor-basedelectronic programmable controller that can provide all the necessarycontrol functions and firing pattern generation needed to implement thepower converter. The power management system can be implemented on anAMC controller. This is a microprocessor-based electronic controllerdesigned for use in distributed control systems. All of these productsare supplied by Converteam Ltd of Boughton Road, Rugby, WarwickshireCV21 1BU, United Kingdom.

The controller arrangement proposes two independent controllers that arecoordinated by means of control signals being sent from the motor bridgecontroller 18 to the network bridge controller 20 and vice versa. Itwould be equally suitable to integrate the functionality of thecontrollers on to one physical controller. Similarly, the functionalitycould be spread across more than two controllers if this is convenientto the practical implementation of the power converter.

The proposed power converter could be used to interface the mainpropulsion drives or the thruster drives of the marine vessel to thesupply network. In either case, the power and/or speed references can beprovided to the controller 24 by a vessel control system. In certaincases, the vessel control system can include a dynamic positioningsystem to provide references to the various propulsion units in order tocontrol the heading and position of the marine vessel.

1. A power converter that can be used to interface a motor that requiresvariable voltage at variable frequency to a supply network providing anominally fixed voltage and nominally fixed frequency, the powerconverter comprising: a first active rectifier/inverter electricallyconnected to a stator of the motor and including a plurality ofsemiconductor power switching devices; a second activerectifier/inverter electrically connected to the supply network andincluding a plurality of semiconductor power switching devices; a dclink connected between the first active rectifier/inverter and thesecond active rectifier/inverter; a first controller for the firstactive rectifier/inverter; and a second controller for the second activerectifier/inverter; wherein the first controller uses a dc link voltagedemand signal indicative of a desired dc link voltage to control thesemiconductor power switching devices of the first activerectifier/inverter to achieve the desired level of dc link voltage thatcorresponds to the dc link voltage demand signal; and wherein the secondcontroller uses a first demand signal to control the semiconductor powerswitches of the second active rectifier/inverter to control the level ofreal power to be transferred into and out of the dc link through thesecond active rectifier/inverter, and a second demand signal to controlthe semiconductor power switching devices of the second activerectifier/inverter to control the level of reactive power and/or acvoltage at the supply network.
 2. A power converter according to claim1, wherein the first controller uses a flux demand signal indicative ofa desired level of flux to be achieved in the motor to control thesemiconductor power switching devices of the first activerectifier/inverter to produce stator electrical quantities that achievethe desired flux in the motor.
 3. A power converter according to claim1, wherein the first controller compares the dc link voltage demandsignal indicative of a desired dc link voltage to a dc link voltagefeedback signal to determine a torque demand signal indicative of adesired level of torque to be achieved in the motor, and controls thesemiconductor power switching devices of the first activerectifier/inverter to produce stator electrical quantities that achievethe desired torque in the motor.
 4. A power converter according to claim1, wherein the first controller supplies a control signal that varies inaccordance with the prevailing motor conditions to the second controllerand the second controller uses the control signal to limit the level ofpower that is transferred to the dc link from the supply network throughthe second active rectifier/inverter.
 5. A power converter according toclaim 1, wherein the first demand signal is based on a motor demand andthe second controller uses the first demand signal to produce desiredmotor electrical and mechanical quantities.
 6. A power converteraccording to claim 1, wherein the second controller uses the firstdemand signal to determine a quadrature axis current demand signal forthe second active rectifier/inverter indicative of a desired quadratureaxis current to be achieved in the supply network, and controls thesemiconductor power switching devices of the second activerectifier/inverter to produce filter/supply network electricalquantities that achieve the desired quadrature axis current in thesupply network.
 7. A power converter according to claim 1, wherein thefirst demand signal is provided by a controller.
 8. A power converteraccording to claim 7, wherein the controller receives one or more of apower reference, a speed reference, torque reference, and a currentreference from a vessel control system.
 9. A power converter accordingto claim 1, wherein the second controller uses the second demand signalto control the semiconductor power switching devices of the secondactive rectifier/inverter to produce desired filter/supply networkelectrical quantities.
 10. A power converter according to claim 1,wherein the second controller uses the second demand signal to determinea direct axis current demand signal for the second activerectifier/inverter, and controls the semiconductor power switchingdevices of the second active rectifier/inverter to produce filter/supplynetwork electrical quantities that achieve the desired direct axiscurrent in the supply network.
 11. A power converter according to claim10, wherein the second controller modifies the direct axis currentdemand signal in accordance with the prevailing supply network voltageconditions.
 12. A power converter according to claim 1, wherein thesecond demand signal is provided by a power management system.
 13. Apower converter according to claim 1, wherein a signal indicative of thesupply network power is supplied to the first controller from the secondcontroller.
 14. A power converter according to claim 13, wherein thesignal indicative of the supply network power is added to the output ofa dc link controller in the first controller and used to determine adesired level of torque in the motor.
 15. A power converter according toclaim 1, further comprising a filter connected between the second activerectifier/inverter and the supply network, the filter including networkterminals.
 16. An arrangement comprising a plurality of power convertersconnected to a common supply bus of a supply network providing anominally fixed voltage and nominally fixed frequency, each powerconverter comprising: a first active rectifier/inverter electricallyconnected to a stator of a motor and including a plurality ofsemiconductor power switching devices; a second activerectifier/inverter electrically connected to the supply network andincluding a plurality of semiconductor power switching devices; a dclink connected between the first active rectifier/inverter and thesecond active rectifier/inverter; a first controller for the firstactive rectifier/inverter; and a second controller for the second activerectifier/inverter; wherein the first controller uses a dc link voltagedemand signal indicative of a desired dc link voltage to control thesemiconductor power switching devices of the first activerectifier/inverter to achieve the desired level of dc link voltage thatcorresponds to the dc link voltage demand signal; wherein the secondcontroller uses a first demand signal to control the semiconductor powerswitches of the second active rectifier/inverter to control the level ofreal power to be transferred into and out of the dc link through thesecond active rectifier/inverter, and a second demand signal to controlthe semiconductor power switching devices of the second activerectifier/inverter to control the level of reactive power and/or acvoltage at the supply network; and wherein the second demand signal issupplied to the second controller of each power converter from a powermanagement system.
 17. An arrangement according to claim 16, whereineach individual power converter includes a step-down transformerelectrically connected between the associated filter and the commonsupply bus.
 18. A propulsion unit comprising: a motor having a statorand rotor; a propeller assembly including at least one blade rotated bythe rotor of the motor; and a power converter for interfacing the motorto a supply network, the power converter comprising: a first activerectifier/inverter electrically connected to the stator of the motor andincluding a plurality of semiconductor power switching devices; a secondactive rectifier/inverter electrically connected to the supply networkand including a plurality of semiconductor power switching devices; a dclink connected between the first active rectifier/inverter and thesecond active rectifier/inverter; a first controller for the firstactive rectifier/inverter; and a second controller for the second activerectifier/inverter; wherein the first controller uses a dc link voltagedemand signal indicative of a desired dc link voltage to control thesemiconductor power switching devices of the first activerectifier/inverter to achieve the desired level of dc link voltage thatcorresponds to the dc link voltage demand signal; and wherein the secondcontroller uses a first demand signal to control the semiconductor powerswitches of the second active rectifier/inverter to control the level ofreal power to be transferred into and out of the dc link through thesecond active rectifier/inverter, and a second demand signal to controlthe semiconductor power switching devices of the second activerectifier/inverter to control the level of reactive power and/or acvoltage at the supply network.
 19. A marine vessel comprising: a supplynetwork providing a nominally fixed voltage and nominally fixedfrequency and having a common supply bus; and a plurality of propulsionunits, each propulsion unit comprising a motor having a stator androtor, a propeller assembly including at least one blade rotated by therotor of the motor, and a power converter for interfacing the motor tothe supply network, the power converter comprising: a first activerectifier/inverter electrically connected to the stator of the motor andincluding a plurality of semiconductor power switching devices; a secondactive rectifier/inverter electrically connected to the supply networkand including a plurality of semiconductor power switching devices; a dclink connected between the first active rectifier/inverter and thesecond active rectifier/inverter; a first controller for the firstactive rectifier/inverter; and a second controller for the second activerectifier/inverter; wherein the first controller uses a dc link voltagedemand signal indicative of a desired dc link voltage to control thesemiconductor power switching devices of the first activerectifier/inverter to achieve the desired level of dc link voltage thatcorresponds to the dc link voltage demand signal; and wherein the secondcontroller uses a first demand signal to control the semiconductor powerswitches of the second active rectifier/inverter to control the level ofreal power to be transferred into and out of the dc link through thesecond active rectifier/inverter, and a second demand signal to controlthe semiconductor power switching devices of the second activerectifier/inverter to control the level of reactive power and/or acvoltage at the supply network; wherein the respective power convertersof the plurality of propulsion units are connected to the common supplybus and wherein the second demand signal is supplied by a powermanagement system.
 20. A method of operating a power converter that canbe used to interface a motor that requires variable voltage at variablefrequency to a supply network providing a nominally fixed voltage andnominally fixed frequency, the power converter comprising: a firstactive rectifier/inverter electrically connected to a stator of themotor and including a plurality of semiconductor power switchingdevices; a second active rectifier/inverter electrically connected tothe supply network and including a plurality of semiconductor powerswitching devices; a dc link connected between the first activerectifier/inverter and the second active rectifier/inverter; a firstcontroller for the first active rectifier/inverter; and a secondcontroller for the second active rectifier/inverter; wherein the methodcomprises the steps of: the first controller using a dc link voltagedemand signal indicative of a desired dc link voltage to control thesemiconductor power switching devices of the first activerectifier/inverter to achieve the desired level of dc link voltage thatcorresponds to the dc link voltage demand signal; and the secondcontroller using a first demand signal to control the semiconductorpower switches of the second active rectifier/inverter to control thelevel of real power to be transferred into and out of the dc linkthrough the second active rectifier/inverter, and a second demand signalto control the semiconductor power switching devices of the secondactive rectifier/inverter to control the level of reactive power and/orac voltage at the supply network.
 21. A method according to claim 20,further comprising the step of the first controller using a flux demandsignal indicative of a desired level of flux to be achieved in the motorto control the semiconductor power switching devices of the first activerectifier/inverter to produce stator electrical quantities that achievethe desired flux in the motor.
 22. A method according to claim 20,further comprising the step of first controller comparing the dc linkvoltage demand signal indicative of a desired dc link voltage to a dclink voltage feedback signal to determine a torque demand signalindicative of a desired level of torque to be achieved in the motor, andcontrolling the semiconductor power switching devices of the firstactive rectifier/inverter to produce stator electrical quantities thatachieve the desired torque in the motor.
 23. A method according to claim20, further comprising the steps of the first controller supplying acontrol signal that varies in accordance with the prevailing motorconditions to the second controller and the second controller using thecontrol signal to limit the level of power that is transferred to the dclink from the supply network through the second activerectifier/inverter.
 24. A method according to claim 20, wherein thefirst demand signal is based on a motor demand and further comprisingthe step of the second controller using the first demand signal toproduce desired motor electrical and mechanical quantities.
 25. A methodaccording to claim 20, further comprising the step of the secondcontroller using the first demand signal to determine a quadrature axiscurrent demand signal for the second active rectifier/inverterindicative of a desired quadrature axis current to be achieved in thesupply network, and controlling the semiconductor power switchingdevices of the second active rectifier/inverter to produce filter/supplynetwork electrical quantities that achieve the desired quadrature axiscurrent in the supply network.
 26. A method according to claim 20,wherein the first demand signal is provided by a controller.
 27. Amethod according to claim 26, further comprising the step of thecontroller receiving one or more of a power reference, a speedreference, a torque reference, and a current reference from a vesselcontrol system.
 28. A method according to claim 20, further comprisingthe step of the second controller using the second demand signal tocontrol the semiconductor power switching devices of the second activerectifier/inverter to produce desired filter/supply network electricalquantities.
 29. A method according to claim 20, further comprising thestep of the second controller using the second demand signal todetermine a direct axis current demand signal for the second activerectifier/inverter, and controlling the semiconductor power switchingdevices of the second active rectifier/inverter to produce filter/supplynetwork electrical quantities that achieve the desired direct axiscurrent in the supply network during a supply network voltage dipsituation.
 30. A method according to claim 29, wherein the secondcontroller modifies the direct axis current demand signal in accordancewith the prevailing supply network voltage conditions during a supplynetwork voltage dip situation.
 31. A method according to claim 20,wherein the second demand signal is provided by a power managementsystem.
 32. A method according to claim 20, further comprising the stepof supplying a signal indicative of the supply network power to thefirst controller from the second controller.
 33. A method according toclaim 32, wherein the signal indicative of the supply network power isadded to the output of a dc link controller in the first controller andused to determine a desired level of torque in the motor.
 34. A methodof operating a plurality of power converters connected to a commonsupply bus of a supply network providing a nominally fixed voltage andnominally fixed frequency, each power converter comprising: a firstactive rectifier/inverter electrically connected to a stator of a motorand including a plurality of semiconductor power switching devices; asecond active rectifier/inverter electrically connected to the supplynetwork and including a plurality of semiconductor power switchingdevices; a dc link connected between the first active rectifier/inverterand the second active rectifier/inverter; a first controller for thefirst active rectifier/inverter; and a second controller for the secondactive rectifier/inverter; wherein the first controller uses a dc linkvoltage demand signal indicative of a desired dc link voltage to controlthe semiconductor power switching devices of the first activerectifier/inverter to achieve the desired level of dc link voltage thatcorresponds to the dc link voltage demand signal; wherein the secondcontroller uses a first demand signal to control the semiconductor powerswitches of the second active rectifier/inverter to control the level ofreal power to be transferred into and out of the dc link through thesecond active rectifier/inverter, and a second demand signal to controlthe semiconductor power switching devices of the second activerectifier/inverter to control the level of reactive power and/or acvoltage at the supply network; the method comprising the step ofsupplying the second demand signal to the second controller of eachpower converter from a power management system.